The invention relates to a cooling arrangement in a frequency converter.
A frequency converter is a power electronics device for steplessly adjusting the rotation speed of an alternating-current motor, such as a short circuit motor. The rotation speed of a short circuit motor is proportional to frequency, so by steplessly changing the frequency of a motor its rotation speed changes correspondingly. A frequency converter may be used for adjusting the acceleration and deceleration as well as the braking and reversing of a motor. Based on its function, a frequency converter may be divided into four different parts:                a rectifier for rectifying a received alternating voltage,        an intermediate circuit for filtering a direct voltage received from the rectifier,        an inverter for changing the rectified and filtered voltage back to an alternating voltage, and        a control unit for controlling the operation of the inverter on the basis of received measurement information to enable a correct frequency to be provided for the voltage to be fed to a motor.        
In such frequency converters power loss occurs, causing by such lost capacity a large amount of heat that should be removed in order to protect the components of the frequency converters. The demand for cooling air of frequency converters varies considerably: for small frequency converters 70 m3/h may suffice, but the demand increases significantly, the largest frequency converters requiring up to 8000 m3/h. In order to protect the frequency converters a transfer means, e.g. a blower, is used, which is arranged to conduct the heat generated in a frequency converter to a space surrounding the frequency converter by means of an air flow.
Previously it has been known to use constant speed fans for cooling electrical devices. This implementation is successful, but the speed of a fan has to be set according to the worst possible situation, which means that the cooling capacity is unnecessarily high. Nowadays a need exists to optimize the efficiency of an electrical device and minimize losses. One way to optimize a device is to try to remove any unnecessary power consumption required for cooling.
In JP60131094 an attempt is made at this by arranging the motor running a cooling fan together with the motor output. In such a case, the speed of the fan operating as a cooler varies together with the motor output. This results in lesser cooling when the speed of the motor is lower and the device needs less cooling. Higher speeds require more cooling, also the fan rotating quicker. In such a cooling arrangement, however, the cooling capacity has to be dimensioned using a certain safety factor since the surrounding conditions are totally ignored during use. The environmental conditions, e.g. the surrounding temperature, around a device may vary up to 50 degrees (−10° to +40°). In many lower temperature cases the power consumption of such a solution is thus unnecessarily high. Thus, a solution wherein cooling is implemented only on the basis of the motor output of a frequency converter does not take the internal temperatures of the frequency converter into account. Neither does the solution take into account a situation where the load is high even at low revolutions. Devices, for instance elevators, exist that require high cooling capacity even at low speeds.